The present invention is generally directed to a method for measuring interfacial stick-slip transitions (SST) and an improved constant-force shear capable of measuring interfacial SST. Some embodiments are capable of measuring SST under simple shear conditions and/or in highly entangled polymer melts. Some embodiments include the application of a constant shearing force to a polymer sample.
Hydrodynamic boundary conditions (HBC) play a crucial role in characterization of flow behavior of various fluids. Since the time of Navier and Stokes (see Navier, Mémo Acad Royal Sci Inst France 6, 414 (1823); Stokes, Trans Cam Phil Soc 8, 299 (1845)), the postulate of no-slip or stick HBC has been brought into question. However, the first piece of evidence for slip manifested on macroscopic scales appears to be the publication E. B. Bagley, I. M. Cabott and D. C. West, J. Appl. Phys. 29, 109 (1958), which is based on capillary flow of high density polyethylene. Polymeric fluids are clearly the only single-component materials that exhibit strong violation of no-slip HBC on macroscopic scales.
Recently there has been growing understanding of polymer slip partially reported in the publication F. Brochard and P. G. de Gennes, Langmuir, 3033 (1992). Two types of experimental activities have been carried out to explore the wall slip behavior of entangled polymers on macroscopic scales, see, for example, Y. X. Zhu, S. Granick, Phys. Rev. Lett. 88, 106102 (2002), involving controlled-pressure capillary flow, as well as P. A. Drda and S. Q. Wang, Phys. Rev. Lett. 75, 2698 (1995); S. Q. Wang and P. A. Drda, Macromolecules 29, 4115 (1996); and X. Yang et al., Rheol. Acta. 37, 415 (1998). S. Q. Wang, Adv. Polym. Sci. 138, 227 (1999) discusses simple shear produced in parallel-plates by controlling the speed of one of the two plates, as does L. Leger, J. Phys.—Condens. Mat. 15, S19 (2003). In the former, a stick-slip transition (SST) was observed at a critical pressure, see P. A. Drda and S. Q. Wang, Phys. Rev. Lett. 75, 2698 (1995); S. Q. Wang and P. A. Drda, Macromolecules 29, 4115 (1996); X. Yang et al., Rheol. Acta. 37, 415 (1998). In the latter, the slip velocity was measured as a function of the plate velocity V and found to have smooth dependence on V, see L. Leger, J. Phys.—Condens. Mat. 15, S19 (2003).
The previous studies involving Dao T. T. and L. A. Archer, “Stick-slip dynamics of entangled polymer liquids,” Langmuir 18, 2616-2624 and J. M. Dealy, presented at IUPAC World Polymer Congress MACRO 2004, involve a sliding plate shear device, operated in the mode of displacing one of the two surfaces with a controlled speed. More recently Dealy proposed that the origin of the previously reported interfacial SST in capillary flow was a system instability, i.e., an experimental artifact related to the design of the controlled-pressure capillary rheometer. An important question now presents itself as follows. Do entangled polymers undergo a discontinuous interfacial stick-slip transition in viscometric flow? Since it is the interfacial shear stress that determines the nature of the HBC for entangled polymers on solid surfaces, this question must be answered by employing a shearing device where the wall stress is the controlling variable. Because all previous studies were based on sliding plate rheometry that operated in the displacement-controlled mode, they inherently could not provide an answer to the above question.